Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/is-regexp/readme.md

@@ -0,0 +1,31 @@
+# is-regexp [![Build Status](https://travis-ci.org/sindresorhus/is-regexp.svg?branch=master)](https://travis-ci.org/sindresorhus/is-regexp)
+
+> Check whether a variable is a regular expression
+
+
+## Install
+
+```sh
+$ npm install --save is-regexp
+```
+
+
+## Usage
+
+```js
+var isRegexp = require('is-regexp');
+
+isRegexp('unicorn');
+//=> false
+
+isRegexp(/unicorn/);
+//=> true
+
+isRegexp(new RegExp('unicorn'));
+//=> true
+```
+
+
+## License
+
+MIT © [Sindre Sorhus](http://sindresorhus.com)